Protected Functions

Detailed Description

A paint device is an abstraction of a two-dimensional space that can be drawn on using a QPainter. Its default coordinate system has its origin located at the top-left position. X increases to the right and Y increases downwards. The unit is one pixel.

To implement support for a new backend, you must derive from QPaintDevice and reimplement the virtual paintEngine() function to tell QPainter which paint engine should be used to draw on this particular device. Note that you also must create a corresponding paint engine to be able to draw on the device, i.e derive from QPaintEngine and reimplement its virtual functions.

Warning: Qt requires that a QGuiApplication object exists before any paint devices can be created. Paint devices access window system resources, and these resources are not initialized before an application object is created.

The QPaintDevice class provides several functions returning the various device metrics: The depth() function returns its bit depth (number of bit planes). The height() function returns its height in default coordinate system units (e.g. pixels for QPixmap and QWidget) while heightMM() returns the height of the device in millimeters. Similiarily, the width() and widthMM() functions return the width of the device in default coordinate system units and in millimeters, respectively. Alternatively, the protected metric() function can be used to retrieve the metric information by specifying the desired PaintDeviceMetric as argument.

The logicalDpiX() and logicalDpiY() functions return the horizontal and vertical resolution of the device in dots per inch. The physicalDpiX() and physicalDpiY() functions also return the resolution of the device in dots per inch, but note that if the logical and physical resolution differ, the corresponding QPaintEngine must handle the mapping. Finally, the colorCount() function returns the number of different colors available for the paint device.

The number of different colors available for the paint device. See also colorCount().

QPaintDevice::PdmDepth

6

The bit depth (number of bit planes) of the paint device. See also depth().

QPaintDevice::PdmDpiX

7

The horizontal resolution of the device in dots per inch. See also logicalDpiX().

QPaintDevice::PdmDpiY

8

The vertical resolution of the device in dots per inch. See also logicalDpiY().

QPaintDevice::PdmPhysicalDpiX

9

The horizontal resolution of the device in dots per inch. See also physicalDpiX().

QPaintDevice::PdmPhysicalDpiY

10

The vertical resolution of the device in dots per inch. See also physicalDpiY().

QPaintDevice::PdmDevicePixelRatio

11

The device pixel ratio for device. Common values are 1 for normal-dpi displays and 2 for high-dpi "retina" displays.

QPaintDevice::PdmDevicePixelRatioScaled

12

The scaled device pixel ratio for the device. This is identical to PdmDevicePixelRatio, except that the value is scaled by a constant factor in order to support paint devices with fractional scale factors. The constant scaling factor used is devicePixelRatioFScale(). This enum value has been introduced in Qt 5.6.

int QPaintDevice::physicalDpiX() const

Returns the horizontal resolution of the device in dots per inch. For example, when printing, this resolution refers to the physical printer's resolution. The logical DPI on the other hand, refers to the resolution used by the actual paint engine.

Note that if the physicalDpiX() doesn't equal the logicalDpiX(), the corresponding QPaintEngine must handle the resolution mapping.

int QPaintDevice::physicalDpiY() const

Returns the horizontal resolution of the device in dots per inch. For example, when printing, this resolution refers to the physical printer's resolution. The logical DPI on the other hand, refers to the resolution used by the actual paint engine.

Note that if the physicalDpiY() doesn't equal the logicalDpiY(), the corresponding QPaintEngine must handle the resolution mapping.